An example of a conventional satellite tuner is described in U.S. Pat. No. 6,031,878. The tuner includes a down-converter for directly converting a received satellite signal to a baseband, to tune to the signal converted to the baseband. Active filters provide a low pass function to reject adjacent channels above the baseband. The filtered signal is fed to a conventional set-top box for processing at the baseband.
Desirable properties of the filter are that it should have a substantially constant group delay (i.e., a constant phase delay characteristic) across at least its pass band, and it should have a steep roll-off (attenuation curve) above its passband. A constant group delay is desired to avoid distortion of pulse shapes and of phase-encoded information. A sharp roll-off is desired in order to ensure rejection of an adjacent higher channel just above the passband. For example, a channel may typically occupy a bandwidth of 30 MHz, and adjacent channels may have their centres at a 40 MHz spacing, leaving only a gap of 10 MHz between adjacent channels. Therefore, the filter should have a flat passband, and a steep roll-off to attenuate the next channel.
By way of example, FIGS. 1 and 2 compare the group delay and the magnitude response for three known filters, a Butterworth filter (curves 10), a Gaussian Response Filter (curves 12) and an Equiripple filter (curves 14). For further information about such filter types, reference may be made to the “Handbook of Filter Synthesis” by Anatol I. Zverev, John Wiley and Sons, 1967.
As can be seen in FIG. 1, the Butterworth filter 10 exhibits significant peaking in its group delay characteristics in the passband, at the cut-off frequency 16. This reduces the phase linearity of the filter. The Gaussian response filter 12 and equiripple filter 14 have a much more desirable flatter group delay, which extends to above the −3 dB frequency 16 for the filter. Gaussian response and equiripple filters are examples of Gaussian family filters, namely a filter which has a substantially flat group delay characteristic up to at least the cut-off frequency of the filter.
However, as can be seen in FIG. 2, a trade-off for the flatter group delay characteristic of the Gaussian response filter 12 and the equiripple filter 14 is that they have a relatively poor roll-off characteristic above the passband. The roll-off characteristics for the Butterworth filter 10 is much steeper, leading to better rejection of an unwanted adjacent channel. For an equiripple filter, the attenuation from the −3 dB frequency to twice this frequency is about 11 dB, which is relatively poor for rejecting an adjacent signal channel. Therefore, it would be desirable to improve the roll-off characteristics of a Gaussian family filter to make such devices more suitable for satellite tuner applications.
In a different field, equiripple filters have found large-scale use in read-channel (data retrieval) integrated circuits, for use in hard disk drives, magneto-optical disc drives, and DVD optical storage systems. The equiripple filter is modified to include high frequency boost, in order to correct the relative amplitude of high frequency signals which tend to be attenuated during the data read operation. By controlling the level of high frequency boost, the attenuated high frequency signals can be restored to the same amplitude as the low frequency signals. Referring to FIG. 3, a conventional filter 20 is a 7th order filter, consisting of an initial 1st order real axis low pass pole (stage) 22, and three 2nd order biquadratic poles 24 characteristic of equiripple filters, each pole having a different and independent −3 dB frequency from the other poles. The high frequency boost is provided by a parallel boost (high pass) path 26 from the 1st order pole 22 to a summing stage 28 downstream of the first of the 2nd order poles 24. The characteristics of this type of filter for use in a data read-channel are well known and documented (for example, Arthur B. Williams and Fred J. Taylor, “Electronic Filter Design Handbook”, McGraw-Hill, 1988).
However, such a high-frequency boosted filter 20 in which all of the input high frequency is boosted by the boost path 26 is only suitable for signals which contain relatively little high frequency content compared to the signal of interest. If the input signal contains significant high frequency content, then that content could overload the boost path 26 and the summing stage 28, causing signal distortion. It would of course be possible to decrease the input amplitude of the signal, but it is generally desired that the input amplitude should be as high as possible for optimum signal-to-noise ratio. Such a decreased input amplitude would also require post-amplification to restore the signal amplitude to a conventional level for other circuits.
FIGS. 4 and 5 show the respective information content in a data read signal 30 from a pick up of a conventional data retrieval device, and a signal 32 from a down converter of a conventional satellite tuner. The data read signal 30 includes a desired frequency band 34 (with attenuated high frequencies to be corrected) and a band of relatively low amplitude high frequency noise 36 to be rejected. In this signal, there is relatively little high frequency information, and the desired frequency band 34 forms the majority of the signal 32, making this a signal for which the filter 20 is suitable (as explained above). In contrast, the down-converted satellite signal 32 includes the desired channel 38, and one or more undesired adjacent higher channels 40 to be rejected. Therefore, this signal 32 includes significant high frequency information (40) of equal amplitude to the desired channel 38, making this a signal for which the filter 20 is generally unsuitable.
Moreover, in the circuit of FIG. 3, the high frequency boost path 26 is provided only for the purposes of equalising the level of high frequencies in the data read signal. Satellite signals do not include such a loss of high frequencies, and so there is no logical use of a high frequency boost path for satellite signals. The use of the same high frequency boost to boost the amplitude of high frequencies in satellite signals would in fact cause amplitude non-linearity in the desired passband. Moreover, high frequency boost is also counter-intuitive in a filter for rejecting an adjacent higher frequency channel which may be close in frequency to the desired channel, and which may have approximately the same amplitude as the desired channel.